Secondary batteries, such as lithium-ion batteries or nickel-metal-hydride batteries, are widely used as power sources in mobile terminal devices that are typically mobile phones. In the recent increasing trend of protection of the environment, attention is being paid to renewable energies, such as solar power and wind power, and secondary batteries are becoming popular and being widely used to store the energies. As for automobiles, hybrid vehicles and electric vehicles that carry secondary batteries are becoming popular. In this manner, secondary batteries serve as key devices essential in power usage.
In such circumstances, accurately sensing the remaining capacity (also referred to as the state of charge (SOC), as appropriate) of a secondary battery is critical in responding to the trust the user has for the product. If the accuracy of sensing SOC is poor in a power storage device for emergencies (such as a power failure), for example, the power is unexpectedly cut off, and a power failure occurs in the power storage device, even though the display indicates a sufficient remaining capacity. In the case of a vehicle, for example, even when the display indicates a sufficient remaining capacity, the vehicle might unexpectedly stop, and fail to reach its destination.
According to a typical method of estimating SOC, open-circuit voltage (OCV) is first estimated, and SOC is estimated from the relationship between OCV and SOC specified beforehand in a table or the like. There is a known method of estimating OCV while updating the parameters of an equivalent circuit model with an adaptive filter or a Kalman filter. Meanwhile, as for the relationship between OCV and SOC (this relationship is also called an OCV curve, as it draws a curved line), it is known that the types of the positive and negative electrodes vary primarily with battery configurations. As the battery degrades, the shape of the OCV curve changes. Accordingly, the SOC estimation accuracy depends on how accurately the OCV curve is calculated.
It is known that an OCV curve has hysteresis characteristics. That is, different OCV curves are obtained when discharge is larger (the proportion of discharge is larger than the proportion of charge) and when charge is larger (the proportion of charge is larger than the proportion of discharge). Therefore, it is necessary to hold two OCV curves for large discharge and large charge. Further, when an OCV curve is used, it is necessary to select one of the OCV curves.
There is a method of selecting the OCV curve to be used, by comparing a cumulative charge current amount with a cumulative discharge current amount (see Patent Document 1). As another method of estimating an OCV curve, there is a method of performing fitting and estimating a degradation state by expanding/contracting and shifting the discharge (charge) curves of the positive electrode and the negative electrode, and adding the result to a discharge (charge) curve history with respect to a small current (see Patent Document 2).